Moonpools are well known in the art, and generally comprise a vertical passageway through a floating structure, often a vessel, through which operations can be directed wholly or substantially vertically, without the need for operating ‘over the side’ of the vessel.
Generally, moonpools have a ‘work table’, i.e. one set of retractable opposing doors movable between a horizontal retracted or open position providing an opening into the moonpool, and a number of closed positions where the opposing faces of the doors are brought closer together, often in a complementary fashion, to reduce the open area thereinbetween and to provide the working area or platform around the reduced opening between the doors. Where the opposing doors completely meet, the moonpool is effectively closed or shut, which would be the normal operation when the moonpool is not required.
The open area between the doors allows the passage of an apparatus, device, unit etc. through the opening. One such apparatus is a conduit, generally an elongate conduit, which is intended either to be laid into the water, generally the sea, or recovered therefrom. A typical example is a pipeline to be used in the production of hydrocarbons, especially an offshore pipeline that is intended to be laid on, in or near the seabed.
In particular, moonpools are useful for the laying of rigid pipelines where the direction of the pipeline from its storage on the vessel into the sea (and then down to its laying position, often in, on or next to the seabed) is critical. Commonly, the direction is guided by one or more guide means, in particular caterpillar tracks or the like, located along a neighbouring tower and in line with the moonpool.
Where the conduit has a regular outer dimension i.e. a constant diameter, the front edges of the moonpool doors can be located closely thereto. Optionally one or more transverse but isolated deflector plates are further operated to buffer against the conduit and maintain it in the desired trajectory envelope as it passes through the moonpool and away from the vessel. Where an accessory is desired to be added to the conduit, or indeed any other change in the shape or circumference of the item passing through the moonpool occurs, the moonpool work table doors can be retracted to increase the size of the moonpool opening to allow the accessory therethrough, prior to being retracted where the diameter of the conduit, etc. is again constant.
Such arrangements and operations are well known in the art. However, existing moonpools have a number of disadvantages. In particular, with the use of isolated extendible supports, there is no tolerance should the conduit move away from the expected trajectory. Crude and usually manual methods are required to retrieve the conduit back into its intended trajectory prior to reapplying the moonpool doors or supports. Secondly, existing moonpools have sharp edges that could damage a conduit or any accessory. Thirdly, the “capture area”, size of the opening within an existing moonpool is relatively small and fixed.